Wireless local area networks are becoming more commonplace as the use of portable computers, such as “laptop,” “notebook,” and “pen” computers become increasingly common in office environments and other locations. In most conventional wireless local area networks, a number of access point base stations form a cellular network for communicating with wireless mobile stations or other mobile devices. Each access point base station is typically connected to a network server, such as part of an ethernet or other network infrastructure. Any messages transmitted as wireless communication signals are first transmitted to an access point base station instead of transmitted along wireless stations. This type of centralized wireless communication using cells provides control over communications along existing wireless mobile devices. Typically, the wireless communication signals are a spread spectrum communications signal, for example, a direct sequence spread spectrum signal, or a frequency hopping spread spectrum signal.
Although wireless local area networks are becoming more commonplace in offices and similar environments, most wireless local area networks do not provide the capability of determining the location of a wireless mobile device or other signal source operating in the wireless LAN environment. Although some wireless LAN systems provide for signal strength analysis of spread spectrum signals to determine location, none of them provide an accurate means of determining the location of a mobile device operative within the wireless infrastructure defined by access point base stations.
Various types of wireless local area network systems that use location systems of the assignee are disclosed in U.S. Pat. Nos. 6,892,054; 6,987,744; and 7,046,657, the disclosures which are hereby incorporated by reference in their entirety. Much of the disclosure in these incorporated by reference patents rely on differential time-of-arrival (TOA) applications that receive a transmitted signal at multiple physical locations corresponding to a receiver to locate a tag transmitter. The clocks could be synchronized so that differences in the indicated time-of-arrival closely approximate the actual time-of-arrival difference. It would be advantageous if the location systems used in a wireless local area network could also determine differences in signal arrival times, for example, when receivers are unsynchronized to aid in locating signal sources such as signaling to receivers, for example, access points.
Commonly assigned U.S. Patent Publication Nos. 2005/0193149 and 2005/0280578, the disclosures which are hereby incorporated by reference in their entirety, are directed to a location system and method that achieves time synchronized network performance using unsynchronized receiver clocks and to achieving synchronized network performance with nodes divided into separate networks. A processor is operative with a receiver and determines clock timing relationships for the clocks based on one of at least propagation or processing delays in the receivers to synchronize receiver performance. These systems are not directed, however, to WLAN systems.